In electric motors and generators, and other commutating electric machinery driven by induced magnetomotive force (m.m.f.), the well-known phenomenon of armature reaction results in reduction of available torque and potential damage to the device's brushes and/or commutator. In the commutator, the cross-magnetization resulting from the magneto-motive force introduces a flux non-uniformity at the pole tip which is reduced while it increases at the opposite pole tip. Due to saturation, the decrease often exceeds the increase, leading to a net flux decrease. The voltage distribution around the commutator has the same shape as the flux wave form and, thus, also becomes irregular. The increased value of the voltage difference between commutator segments promotes sparking, arcing, or "flash-over".
Machines which are subjected to abrupt load changes are particularly susceptible to such sparking. To reduce the risk of flash-over, additional poles, or windings, are commonly incorporated into the commutator, connected in series with the armature to balance the armature m.m.f. An added advantage is that the inductance of the two windings is less than that of the armature alone, so that the sudden change of armature current is less severe. However, the addition of poles has the disadvantages of increasing the overall size and complexity of the machine, and the output power is decreased.
Various inventions have been addressed to suppression of sparking COMMUTATING electrical machines, including the use of resistance paths (U.S. Pat. No. 3,487,248 of Kaneko et at.) or quenching capacitors (U.S. Pat. No. 3,529,589 of Schaub) between commutator sections, conductive spark-quenching grease for coating the commutator sections and filing the gaps therebetween (U.S. Pat. No. 4,319,153 of Mabuchi), conductive sheets placed in flux zones to produce eddy currents which oppose flux changes (U.S. Pat. No. 3,409,788 of Taylor), resistors connected in series with brush elements (U.S. Pat. No. 3,456,143 of Uemura et al.), and a combination of an insulating washer and conductive ring adjacent to the commutator (U.S. Pat. No. 4,734,607 of Ikawa et al.).
For various reasons, the methods introduced by these patents for reducing sparking in electric machinery have not proven acceptable for most applications. Most motors and similar machines are still made which experience sparking problems, as a truly effective method of reducing sparking is not available at present. Efforts have been made to short out the coils if the reactance voltage exceeds a certain threshold. These bypass circuits include those using thermistors to establish a graduated threshold, and those which bypass the commutator segments with gas discharge tubes. Both of these methods are partially effective, and reduce arcing somewhat, but are too slow in response to be truly effective in not only reducing sparking but also increasing torque output, for which a slight delay is productive.
None of the prior art devices use zener diodes as the operative elements in spark reduction. The various prior art references set forth the following elements that ostensibly eliminate or reduce the arcing problem by connecting to the incoming ends of the windings:
A quenching capacitor; PA1 A spark quenching grease coat; PA1 A metal sheet in a flux zone to dissipate induced eddy currents; PA1 Resistors; PA1 Varistors (many); PA1 Rotating varistor disc; PA1 Rotating reactance disc; and PA1 Electrically conductive rubber ring.
Thus, on the one hand, there are many motor surge supressors, although none of them uses zener diodes. On the other hand there are many circuits using zener diode voltage caps, but used for non-motor purposes. With so many zener diodes used as AC waveform clippers, one might wonder why there is not a single prior art motor using zener shunts across the windings.
It is applicant's belief that there has been an avoidance of Zeners for use in this capacity because they have no inherent overvoltage protection and are thus subject to runaway current and incineration. This is not a problem, however, as in practice the small intermittent overvoltage conditions do no harm, and if it were perceived as a problem a minimal resistor could remedy the situation.
Thermistor shunts probably come the closest to zener diode shunts in electrical response. However, zeners have no debilitating disadvantages compared to thermistors, and there are substantial advantages when used in anti-flashover commutator segment shunts. Diodes have a high resistance to shock, and military specification models withstand temperatures of over 200 degrees Celsius without damage. Both of these characteristics are considerably better than the varistor counterparts, and ideally suit the zener diode for the motor commutator application due to the constant vibration, shock, and heat.
In addition, varistors have an inherently limited lifespan because every spark destroys a microscopic piece of it. Diodes on the other hand have an indefinite lifespan, and could conceivably work for a hundred years at full capacity provided overvoltages beyond those expected were not experienced. And zener diode shunts improve efficiency. Sample test results on a dynamometer show a definite, statistically meaningful, improvement of 1% to 3% in efficiency. Applicant has run his motors long enough to know that at least with small motors durability is not a problem. Perhaps it is a belief to the contrary that has stifled development. That a government research agency has indicated the arrangement will not work suggests the possibility that inaccurate folklore may have obstructed development, teaching away from the invention.